This invention concerns recovery of a solvent, especially a hydrofluorocarbon (HFC) solvent used in the exaction of components from materials of natural origin. Herein such materials are termed xe2x80x9cbiomassxe2x80x9d and the extraction of such components xe2x80x9cbiomass extractionxe2x80x9d.
The extraction of flavours, fragrances or pharmaceutically active components from materials of natural origin using chlorine-free solvents based on hydrofluorocarbons is of growing technical and commercial interest. In order to avoid the undesirable release of such solvents to atmosphere, the HFC-based solvents are normally utilised in a closed-loop extraction system configuration.
By the term xe2x80x9chydrofluorocarbonxe2x80x9d we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus chlorine-free.
Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly the C1-4 hydrofluoroalkanes. Suitable examples of C1-4 hydrofluoroalkanes which may be used as solvents include, inter alia, tifluoromethane (R-23), fluoromethane (R-41), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1-trifluoroethane (R-143a), 1,1,2,2-tetrafluoroethane (R-134), 1,1,1,2-tetrafluoroethane (R-134a), 1,1-difluoroethane (R-152a), heptafluoropropanes and particularly 1,1,1,2,3,3,3-heptafluoropropane (R-227ea), 1,1,1,2,3,3-hexafluoropropane (R-236ea), 1,1,1,2,2,3-hexafluoropropane (R-236cb), 1,1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane (R-245fa), 1,1,2,2,3-pentafluoropropane (R-245ca), 1 1,1,2,3-pentafluoropropane (R-245eb), 1,1,2,3,3-pentafluoropropane (R-245ea) and 1,1,1,3,3-pentafluorobutane (R--365mfc). Mixtures of two or more hydrofluorocarbons may be used if desired.
R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.
An especially preferred hydrofluorocarbon for use in the present invention is 1,1,1,2-tetrafluoroethane (R-134a).
There are broadly three ways in which the solvent can be managed in such a system;
(a) Once through. A fresh batch of solvent is used for each campaign of biomass extraction in order to minimise inter-product contamination through the solvent or a build-up of undesirable residues within the solvent.
(b) Dedicated solvent. A separate batch of solvent is maintained for each type of biomass to be extracted in order to minimise inter-product contamination through the solvent.
(c) In-situ recovery and recycle. The solvent is recovered and recycled for use between batches of extractions and/or between extraction campaigns.
Option (c) has a number of advantages over (a) and (b), in particular;
Minimisation of the costs of waste solvent disposal through extended processing life.
Minimisation of solvent transport between the site of extraction and a reprocessing facility.
Minimisation of pressurised solvent storage at the extraction site.
all of which are likely to contribute to improved cost-effectiveness for the products of the extraction process. Clearly, in order to implement option (c) an effective and reliable method of ensuring an acceptably low level of inter-batch contaminants is needed. To be effective, the method needs to be capable of removing a wide range of possible organic contaminants from the HFC-based solvent and to dry the solvent prior to storage between extractions.
In extractions where the solvency properties of a single HFC solvent are not capable of providing the desired product in appropriate yield or purity or where the physical properties of the HFC are unfavourable, then the use of a solvent mixture may be required. Typically, these solvent mixtures may be based on blends of HFCs (e.g. R-134a, R-227ea, R-32, R-125 and R-245ca) or on mixtures with essentially co-boiling solvents (e.g. R-134a/dimethyl ether, R-134a/butane or R-134a/CO2). Ethanol represents the most significant member of a third group of co-solvents or entrainers that may be of technological importance in the extraction of materials with HFC-based solvents.
A problem associated with all of these mixtures in a solvent re-use application as described above is that of ensuring a reproducible starting composition for the solvent mixture.
FIG. 1 is a schematic representation of a typical closed-loop solvent extraction cycle.
In the FIG. 1 apparatus 10 biomass is packed into an extraction vessel 11 connected to a closed-loop circuit comprising, in series, a filter 12, a separator in the form of an evaporator 14, a compressor 16 and a condenser/liquid receiver 17.
In use of apparatus 10 a liquid HFC solvent passes through biomass in extraction vessel 11, removing the preferred components therefrom. The liquid solvent/extract mix passes to evaporator 14 where the solvent is evaporated and the preferred components are collected. The preferred components may be e.g. in liquid form, or could be pastes, solids or take other physical forms. Compressor 16 and condenser 17 compress and condense the solvent before returning it to extraction vessel 11 to remove further preferred components from the biomass therein.
Careful distillation of the solvent from the evaporator into the condenser/liquid receiver is likely to result in the removal of the majority of the contaminants from the extraction but in the absence of a properly designed distillation apparatus it is unlikely to be completely effective, resulting in solvent contamination.
According to a first aspect of the invention there is provided a method as defined in Claim 1.
This method is advantageously effective at recovering pure HFC solvent. The process may be conducted repeatedly by recycling the solvent through adsorbent and desiccant materials several times until the desired levels of contaminants and water is achieved. The purification process may be conducted within the circuit of the extraction apparatus, thus acting to continuously solvent-wash the extraction equipment, or in equipment outside of the extraction loop. The aspects of the invention are defined in Claims 2 to 4.
Preferably the adsorbent is carbon-based. More preferably the adsorbent is or includes an activated carbon derived from plant materials such as coconut husk, or from pyrolysis of fossil fuel materials.
Conveniently the desiccant is selected from one or more of aluminosilicate molecular sieves; silica gel; and alumina. Preferably the desiccant is or includes a combination of an aluminosilicate molecular sieve with silica gel and/or alumina. In such a method the molecular sieve advantageously polishes water after gross water removal by the bulk of the silica or alumina.
Preferably the alumina, when present, contains basic sites. These advantageously reduce acidic organic components from the solvent.
Alternatively the alumina, when present, contains acidic sites. These tend to reduce the levels of basic organic contaminants.
The preferred pore sizes in the aluminosilicate molecular sieves used for drying are in the range of 2 xc3x85 to 4 xc3x85.
The adsorbent and desiccant materials, when both are used, may be within a single container or in a plurality of individual containers.
Whilst the processing described above will be suitable for HFC mixtures and for mixtures of HFCs with co-boiling components, the large disparity between the physical properties of the HFC and entrainer solvents in the third group necessitates a different approach.
Solutions to this further problem are defined in Claims 12 to 17. Thus it is proposed that the most appropriate way of providing a reproducible starting solvent composition is to remove the entrainer solvent from the HFC fluid at the end of the extraction and to re-introduce the entrainer in a controlled manner at the front-end of the process. For ethanol, the bulk of the entrainer will be retained in the solvent evaporator 14 along with the extract product requiring additional ethanol to be added to the HFC solvent prior to entry into the extraction vessel. During the extraction cycle, the quantity of entrainer returning from the evaporator is not likely to result in any significant problems since the feed rate of added entrainer can be adjusted if desired. If the evaporation temperature is sufficiently high, a significant quantity of ethanol will circulate in the HFC solvent at the end of the extraction. If the levels of ethanol in the HFC are high, they are likely to interfere with the function of both the organic contaminant adsorbent and the desiccant. Under these circumstances, the excess ethanol may be removed by washing the HFC solvent with water prior to the contaminant removal and drying process described above. This water wash can be accomplished by passing the HFC vapour through a pool of water, through a hydrophilic filter material (e.g. cellulose) moistened with water or by washing the liquid HFC with water followed by decantation.
A further advantage of the method of the invention is that it allows an HFC-based mixture to be circulated around the system 10 in order to act as a cleaning fluid between extraction campaigns. Any contaminated entrainer solvent will accumulate in the evaporator and water wash with the HFC component recovered for re-use.
According to a second aspect of the invention there is provided apparatus as defined in Claim 18. Optional features of the apparatus are defined in Claims 19 to 22.
The apparatus of the invention is advantageously suited for practising of the method steps defined herein.
There now follows a description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the accompanying drawings in which: